Recursive quick sort in python

Question

I was wondering if there is anything that I could do to make my code look cleaner and more descriptive for educational purposes?

def quick_sort(array, start, end):
 if start < end:
 pivot_index = partition(array, start, end)
 quick_sort(array, start, pivot_index - 1)
 quick_sort(array, pivot_index + 1, end)
def partition(array, start, end):
 pivot_index = start
 pivot = array[end]
 for loop_index in range(start, end):
 if array[loop_index] <= pivot:
 array[pivot_index], array[loop_index] = array[loop_index], array[pivot_index]
 pivot_index += 1
 array[pivot_index], array[end] = array[end], array[pivot_index]
 return pivot_index

Answer 1

• If you want your code cleaner and more descriptive, try adding some English text somewhere. I usually find docstrings are the right place to start in python. What does partition do? What does quick_sort do? In general by "what does X do?", I mean what does it take in as input, and what does it return--the reader of a docstring doesn't care what happens in between. Read the standard library function descriptions for nice examples.
• Right now there is no clear entry point--quick_sort takes in three values, which is not what a user would like. Add a function which takes in an array and returns the sorted array. You could either add a new function, or default values for the extra two arguments of quick_sort. Both are common.

Answer 2

Code is clear and straight forward to read. However, there are two assumptions here

1. It will be used with data types for which "<=" operation is defined. It is good idea to extract this part comparator and let caller decide how two members should be compared.
2. array[end] is always good pivot. This may not be always true. Good pivot is critical for quick sort effectiveness. Input randomisation is commonly used to make good pivot selection more probable.

• Peilonrayz: Could you expand on what "Input randomisation" is. I've not come across this before and don't understand what you mean by your answer as is. I feel I could learn something here, but I'm just not able to connect the dots.
• Farzin: @Peilonrayz Here, this might help you.